Consistently creating a final, cooked product appearance that resembles one typically created in a residential oven has proven to be a far more difficult problem to solve in spiral ovens than it is in linear ovens. Linear ovens cook food using an airflow process known as impingement cooking. In impingement cooking, hot vertical airflow impinges directly upon each individual product flowing along the cook belt of the oven. Impingement cooking produces an acceptable “browned” appearance but comes at the price of increased floor space requirements. For example, a 600-foot cook belt requires 600 feet of linear oven.
Spiral ovens solve the floor space problem associated with high volume capacity cooking. A 600-foot cook belt, for example, can be compressed into a 20′×20′×20′ box. However, compressing the cook belt creates problems for the heat transfer system needed to cook the product. In a linear oven, the upper surface of each product is exposed to the impingement cooking system. In a spiral oven, indirect cooking methods must be used because the upper (lower) surface of product within the spiral is hidden by the layer of cook belt and product suspended above (below) it. As a result, a product cooked in a spiral oven in no way resembles the color of the product illustrated on the package in which the product is sold. For example, a microwaveable-ready meal of macaroni-and-cheese or au gratin potatoes may appear on the package as having a golden brown finish. The actual product finish is white and anemic-looking. Microwaving by the consumer to heat the product will not change the color.
Prior art spiral ovens (see FIGS. 1 to 4) have adopted various ways to uniformly distribute and transfer heat to the product being cooked on the belt. One prior art spiral oven (FIG. 1) employs a thermal coil arrangement at one end of the oven and a fan and vent system arranged to create airflow both vertically through the belt mesh of the spiral conveyor and horizontally through the side-links of the conveyor. The fan operates in a single direction of rotation. An example of this airflow system and method is found in JBT FoodTech's (Chicago, Ill.) Stein GYROCOMPACT® II-1000 Oven.
Another prior art spiral oven (FIG. 2) employs a first, lower section cooking zone in which heat is provided by horizontal airflow and a second, upper section cooking zone in which high-velocity hot air impinges vertically on the product being conveyed. The fan operates in a single direction of rotation. An example of this airflow system and method is found in CFS's (The Netherlands) COOKSTAR® three-zone, double-spiral cooker.
Yet another prior art spiral oven (FIG. 3) employs a cylindrical fan and air circulation system arranged to circulate air around all sides of the spiral conveyor and create airflow both horizontally across the conveyor and vertically upward through the belt mesh of the spiral conveyor. The fan operates in a single direction of rotation. An example of this airflow system and method is found in Heat and Control, Inc.'s (Hayward, Calif.) Model SO Twin Drum Spiral Oven.
A final prior art spiral oven (FIG. 4) employs a fan and air circulation system arranged to provide an airflow pattern that impinges upon the outer edge of the belt and flows horizontally across and vertically through the belt mesh of the spiral conveyor. The fan operates in a single direction of rotation. An example of this airflow system and method is found in Unitherm Food Systems, Inc's (Bristow, Okla.) spiral oven.
All prior art spiral ovens are designed to provide a substantially uniform airflow across the cook belt because conventional wisdom in the art of spiral oven design holds that a non-uniform airflow across the belt is not desirable and must be avoided.